1. Technical Field
The present disclosure disclosed herein relates to a nitrogen-doped transparent graphene film and a manufacturing method thereof, and more particularly, to a transparent graphene film which is prepared by maintaining the primary reduced state of a graphene oxide thin film via chemical reduction, reducing the graphene oxide thin film with chemical vapor deposition, and doping nitrogen, thereby enhancing the conductivity and enabling the control of work function and a manufacturing method thereof.
2. Description of the Related Art
Graphene refers to a two-dimensional monolayer structure wherein six-carbon hexagons are connected to each other, and is different from zero-dimensional fullerene, carbon nanotube having a tube-shaped one-dimensional structure, and graphite having a three-dimensional structure. Graphene shows the same behavior as massless Dirac fermions and exhibits the electron mobility of 15,000 cm2V-1s-1 or more at room temperature. It was reported that the tensile strength of graphene is twice or more than that of diamond and 200 to 300 times greater than that of steel. In addition, since graphene is a single carbon layer, it has excellent transparency. Various application possibilities of graphene are being forecasted, owing to its excellent electrical, mechanical, and optical properties.
In order to manufacture graphene, various methods have been suggested and a mechanical method, chemical vapor deposition (CVD), and a chemical method have been reported as representative manufacturing methods.
A mechanical method is to exfoliate graphene from graphite using a sticky tape and has advantages that the size of the manufactured graphene is large about from tens to hundreds of micrometers and intact graphene of which natural properties are not damaged can be obtained. However, it has a limitation in mass-production and has a disadvantage that only graphite can be a source.
Chemical vapor deposition is a method by the chemical reaction of vapor phase components to form a graphene thin film on the substrate surface deposited with certain metals and it has been recently reported in academics. Using this method, graphene with relatively small defects can be obtained. However, the production cost is high and graphene can be grown only on a specimen deposited with certain metals, and a process to transfer grown graphene to desired substrate is required.
A chemical method is a method to obtain graphene by treating graphite with a strong oxidizing agent to form graphene oxide, followed by chemical or thermal reduction of graphene oxide. For chemical reduction, the reduction reaction is induced by the addition of a reducing agent such as hydrazine (N2H4) or potassium hydroxide. For thermal reduction, the reduction reaction is induced by high heat treatment in vacuum or hydrogen atmosphere. Using this method, graphene can be produced inexpensively in large quantities. Using functional groups of graphene oxide which is produced during the making process of graphene, other functional groups or metal particles can be added so that the application is more extensive. However, a chemical method has disadvantages that graphene produced by this method has many defects and its electrical property is poor than that of natural graphene.
As stated before, graphene can be applied for various fields because it has excellent optical, mechanical, and electrical properties. Particularly, attempts to use graphene for transparent electrodes are being made. Methods of using graphene for transparent electrodes start from manufacturing methods for graphene stated before. Therefore, manufacturing methods for transparent electrodes can also be classified in the same way as manufacturing methods for graphene, and advantages and disadvantages of manufactured electrodes also should follow those advantages and disadvantages as enumerated in manufacturing methods for graphene. First, a method to obtain graphene by the mechanical exfoliation of graphite may be quite a useful way to do basic physical research; however, it is excluded from manufacturing methods for electrodes because mass-production and application to large area substrate are impossible.
Second, chemical vapor deposition has an advantage that it can produce graphene with relatively small defects; however, it has limitations that the process can be made only on certain metal substrate and the cost is highly expensive and the secondary defects may be induced during the process of transferring synthesized graphene to desired substrate. Finally, there is a way to produce graphene films in large quantities via a chemical method of making graphene oxide substance from graphite, followed by effective reduction methods. However, while this method has an advantage that mass-production can be accomplished, it has limitations that during the production of graphene oxide from graphite, many defects are produced by strong acid treatment and the graphene film cannot exhibit excellent electrical property which is the characteristic of natural graphene.